This invention relates to a carrier reproducing circuit for Phase shift keying signals for reproducing a carrier which is a part of the Phase shift keying signal.
In the present civilized world, there have been strong demands for quick, various, high quality information services. To meet such demands, industry has made remarkable advances in communication technology, particularly related to information services, data, picture, sound, etc. Typical examples of the implementation of state of the art communication technology include multichannel sound broadcasting, teletext, facsimile, still picture broadcasting, high quality TV, satellite broadcasting, etc. To effectively utilize transmission lines, it is a common practice to employ digital coding transmission for multichannel transmission.
In the satellite broadcasting system, for example, a GHz band signal is radiated from a ground station, and a satellite repeats the signal toward a number of earthbound receiving stations. In this broadcasting system, a channel group consisting of one hundred and several tens channels is used for television signal transmission. Each channel has a 6 MHz frequency band.
For transmitting a sound signal by using the channel group system, the 6 MHz frequency band is divided into five narrow frequency bands, 1st to 5th channels, which are respectively assigned for different sound services. One of the sound signal transmission systems of this type is a phase shift keying (PSK) modulation system. In this PSK modulation system, the phase of a carrier signal is modulated by a binary code signal. A reference phase is set up in the carrier signal. Phases shifted from the reference phase are assigned to binary codes. In the receiving side, a carrier contained in the incoming RF signal is reproduced using the baseband signal. Then, the reproduced carrier is appropriately processed to demodulate the original sound signal.
Since the PSK modulation system has a good frequency utilizing efficiency, the C/N ratio, i.e., subcarrier power to noise ratio, can be large. Particularly, the 4-phase PSK modulation system is excellent in the frequency utilizing efficiency, and easy in bit error correction.
Specifically, in the 4-phase PSK modulation system, the phase of the carrier is shifted to 4 phases, 0.degree., 90.degree., 180.degree., and 270.degree. according to a sound signal. These are subjected to the logical sum operation. Then, the sound data is gray coded. The gray binary code system is featured in that bit error correction is easy because in the gray binary code representation only one bit differs between the adjacent bits, that is a data to data distance becomes 1.
At the receiver, a baseband signal is selected by a phased locked loop (PLL) circuit including a voltage controlled oscillator (VCO), to select a desired channel group. Further, one sound channel is selected from the selected channel group, to effect the 4-phase demodulation of the sound signal. To demodulate the 4-phase PSK signal for sound data transmission, after the carrier is reproduced for phase discrimination, data is demodulated. In this case, the carrier must exactly be reproduced; otherwise bit errors occurs in the reproduced signal, resulting in incorrect data reproduction. To establish the best of the 4-phase PSK modulation system featured by high C/N ratio and easy bit error correction, frequency variation must be minimized during the process of the frequency conversion for carrier reproduction. To reduce the bit error in the reproduced data, a VCO with little frequency variation must be used for that of the carrier reproducing circuit of the 4-phase PSK demodulator.
The frequency variation of the frequency converting means for obtaining the baseband signal and an improper or narrow pull-in frequency range of the VCO would cause the bit error in the reproduced signal. In the case of sound data, so bit error appears in the form called click noise.
When the 4-phase PSK signal is synchronously detected for data demodulation, an error rate Pe of data is given by an equation (2), using the Gaussian distribution function expressed by an equation (1), on the assumption that Gaussian noise of a variance .alpha..sup.2 is superimposed on the synchronous detected output signal. EQU .phi.(z)=1/.sqroot..pi.2 .sub..alpha..sup.Z exp[-t.sup.2 /2]dt . . . (1) EQU Pe=2[1-.phi.(.sqroot.C/N)]. . . (2)
As seen from the equation (2), as the C/N ratio is larger, the bit error rate becomes smaller. To increase the C/N ratio, the pull-in range of the VCO for carrier reproduction need only be widened.
As described above, to reduce the bit error rate, it is necessary to minimize frequency variation during the frequency conversion for baseband signal discrimination and to set up a proper pull-in range of the VCO for carrier reproduction.